In the optical communications space, receivers based on coherent detection techniques have suffered disadvantages that have, to date, prevented successful deployment in “real-world” installed communications networks.
For example, coherent optical receivers tend to be highly sensitive to optical impairments of the received carrier signal. Optical signals received through conventional optical links are typically distorted by significant amounts of chromatic dispersion (CD) and polarization dependent impairments such as Polarization Mode Dispersion (PMD), polarization angle changes and polarization dependent loss (PDL). Chromatic dispersion (CD) on the order of 30,000 ps/nm, and polarization rotation transients at rates of 105 Hz are commonly encountered.
Coherent optical receivers also tend to be highly sensitive to frequency mismatch between the receiver's local oscillator (LO) and the carrier of the inbound optical signal. The transmitted carrier signal and the receiver's local oscillator (LO) signal are generated by respective transmitter and LO lasers, which, in the case of “real world” network systems, will be compact fiber or semi-conductor lasers which are subject to manufacturing and environmental variations. Such lasers are typically designed such that the average output frequency (over a period of 100 s of milliseconds or more) is stable at a value which is nominally fixed by the frequency setting. However, short period frequency excursions due to laser line width and phase noise are permitted. As a result, frequency variations of as much as ±400 MHz, at rates on the order of up to 50 KHz are commonly encountered. The resulting frequency mismatch Δf between the LO signal and the received carrier signal appears as a phase error in recovered symbols, which can lead to erroneous data detection.
Various methods and systems intended to address some of these limitations are known in the art. For example, a method of compensating polarization angle impairments are described in PLL-Free Synchronous QPSK Polarization Multiplex/Diversity Receiver Concept with Digital I&Q Baseband Processing, R Noé, IEEE Photonics Technology Letters, Vol. 17, No. 4, April 2005. In the introduction of this same paper, Noé also alludes to the possibility of also compensating chromatic dispersion, but provides no further discussion as to how this might be done. A method of electronic carrier recovery is described in Phase Noise-Tolerant Synchronous QPSK/BPSK Baseband-Type Intradyne Receiver Concept With Feedforward Carrier Recovery, R Noé, Journal of Lightwave Technology, Vol. 23, No. 2, February 2005. The applicability of RF channel estimation techniques to the detection of polarization-division multiplexed optical signals in a quadrature coherent receiver is described by Y. Han et al. in Coherent optical Communication Using Polarization Multiple-Input-Multiple-Output, OPTICS EXPRESS Vol. 13, No. 19, pp 7527-7534, 19 Sep. 2005.
Frequency Locked Loop (FLL) and Phase Locked Loop (PLL) circuits for compensating the frequency mismatch Δf are described in: High Capacity Coherent Lightwave Systems, Linke et al, Journal of Lightwave Technology, Vol. 6, No. 11, November 1988; Heterodyne Phase Locked Loop by Confocal Fabry-Perot Cavity Coupled AlGaAs lasers, Shin et al, IEEE Photonics Technology Letters, Vol. 2, No. 4, April 1990; and Carrier Synchronization for 3 and 4-bit-per-Symbol Optical Transmission, Ip et al, Journal of Lightwave Technology, Vol. 23, No. 12, December 2005. All of these systems operate to drive the receiver's LO to precisely track excursions of the received optical carrier. A limitation of this approach is that for optical communications systems with multi-gigabit line rates, a PLL/FLL loop bandwidth on the order of hundreds of MHz is needed to effectively compensate the laser phase noise. This is difficult to achieve at acceptable cost.
All of these systems depend on accurate recovery of a clock signal from the received optical signal. The two principle techniques used for this purpose are described by Noé (Supra, April 2005). These include an electronic clock recovery block inserted into the main data path between the photodetectors and the A/D converters, or alternatively an intensity modulation direct detection receiver which recovers a clock signal from light tapped from the input optical fiber. Both of these techniques are highly sensitive to Inter-Symbol-Interference due to chromatic dispersion, and Polarization Mode Dispersion (PMD).
A limitation that is common throughout the prior art is a lack of satisfactory bandwidth of the various compensation functions. For example, the FLL/PLL and carrier recovery techniques described above are intended to track (and thus compensate) laser phase noise. However, in order to provide sufficient accuracy of compensation, they lack sufficient bandwidth to acquire a signal across the entire possible range of impairment magnitude, such as a frequency error of several GigaHertz. As a result, these systems cannot reliably acquire a signal and stabilize to steady-state operation, even if they could track laser phase transients after a steady state had been achieved.
Similarly, the system of Noé (supra, April 2005) is designed to compensate polarization rotations, but it cannot track high speed transients of the type encountered in real-world communications networks. For example, Noé, claims that with a 10 GBaud signal, the inverse Jones matrix coefficients can be updated with a period of 16 μs. This is far too slow to successfully compensate 20 kHz polarization rotations, which have a period of 50 μs. In addition, the system of Noé tends to fail in the presence of severe Chromatic Dispersion (CD), at least in part due to failure of the clock recovery circuit as inter-symbol interference (ISI) increases, and consequent uncertainty of the sample timing of the A/D converters. While it is mathematically possible to design a filter function that compensates both polarization and chromatic dispersion (as alluded to by Noé), the prior art does not offer any methods by which satisfactory compensation accuracy can be obtained with an adaptation speed high enough to track real-world polarization transients. It follows that the system of Noé will not be able to reliably capture the instantaneous polarization state of the received signal during start-up, especially in the presence of high speed transients, and thus cannot guarantee that it will achieve a stable steady-state operation.
Prior art clock recovery systems suffer the same limitation, in that the PLL bandwidth required to obtain a satisfactory sample phase accuracy is significantly less than the possible range of clock and channel errors. As a result, conventional clock recovery circuits cannot reliably acquire a lock condition, even if they are able to maintain lock once it has been achieved. A further limitation of clock recovery circuits is that they are highly vulnerable to distortions in the received optical signal. While this can be overcome by compensating at least some of the distortions prior to the clock recovery circuit, such compensation normally requires the recovered clock signal in order to operate. As a result, the receiver cannot reliably acquire signal and achieve a steady state operation, even if such a state can be maintained once it has been achieved.
Accordingly, combining the above techniques leads to a coherent optical receiver that are capable of compensating: polarization angle variations, but only in the presence of low CD and PMD, and even then only when the polarization angle transients are relatively low speed; frequency mismatch Δf, but only within a frequency range that is too narrow to enable reliable signal acquisition; and dispersion, but only in the presence of low speed polarization transients. Vulnerability of the clock recovery systems to CD, PMD and polarization angle transients compound these limitations.
Applicant's co-pending U.S. patent application Ser. Nos. 11/294,613 filed Dec. 6, 2005 and entitled “Polarization Compensation In A Coherent Optical Receiver”; 11/315,342 filed Dec. 23, 2005 and entitled “Clock Recovery From An Optical Signal With Dispersion Impairments”; 11/315,345 filed Dec. 23, 2005 and entitled “Clock Recovery From An Optical Signal With Polarization Impairments”; 11/366,392 filed Mar. 2, 2006 and entitled “Carrier Recovery In A Coherent Optical Receiver”; and 11/423,822 filed Jun. 13, 2006 and entitled “Signal Acquisition In A Coherent Optical Receiver”, the content of all of which are hereby incorporated herein by reference, describe methods and systems of reliable signal acquisition, clock recovery and polarization compensation in the presence of moderate- to severe optical impairments of a received optical signal. A feature of all of these systems is that a multi-bit “raw” sample stream of the received optical signal is digitally processed to at least partially compensate dispersion, before any of the clock recovery, polarization compensation, or carrier recovery methods are implemented. Since the raw sample stream cannot be inverse multiplexed into parallel substreams without loss of information, this digital processing must be performed at the full sample rate of the data path. In embodiments where Nyquist sampling of the input optical signal is used, this sample rate will be about double the highest expected symbol rate of the optical signal. Nominal symbol rates of 10 Gbaud or higher are anticipated.
Additionally, it is desirable to process the “raw” sample stream to facilitate clock recovery and frame detection functions in the presence of significant amounts or residual dispersion. It is also desirable to process the “raw” sample stream to produce a dispersion compensated sample stream, which retains a proper phase alignment between samples, so as to preserve the original phase information of the received optical signal needed to support downstream polarization compensation and carrier recovery functions. It is further desirable to implement all of the above digital signal processing within an efficient package so as to minimize Integrated Circuit (IC) area and heat generation during operation.